BP blamed a "sequence of failures involving a number of different parties" for the spill.

It faces billions of dollars worth of compensation claims over the disaster.

Transocean dismissed BP's report, accusing the oil giant of having designed a "fatally flawed" well and making "cost-saving decisions that increased risk - in some cases, severely".

"This is a self-serving report that attempts to conceal the critical factor that set the stage for the Macondo (well) incident: BP's fatally flawed well design," the Swiss-based group said in a statement.

Halliburton also hit back at the 193-page report.

"As we continue to review BP's internal report published earlier today, we have noticed a number of substantial omissions and inaccuracies in the document," it said.

"Halliburton remains confident that all the work it performed with respect to the Macondo well was completed in accordance with BP's specifications for its well construction plan and instructions, and that it is fully indemnified under its contract for any of the allegations contained in the report."

Well capped

The rig exploded in April, killing 11 people, and an estimated 4.9m barrels of oil then leaked into the Gulf.

The well was capped on 15 July, and an operation to permanently seal it is due to take place in the coming weeks.

BP leased the Deepwater Horizon rig from Transocean, and its cement contractor was Halliburton. The BP report was critical of the processes and actions of teams from both firms.

The report, conducted by BP's head of safety, Mark Bly, highlighted eight key failures that, in combination, led to the explosion.

BP said that both BP and Transocean staff incorrectly interpreted a safety test which should have flagged up risks of a blowout.

"Over a 40-minute period, the Transocean rig crew failed to recognise and act on the influx of hydrocarbons into the well" which eventually caused the explosion.

BP criticised the cementing of the well - carried out by Halliburton - and repeated previous criticism of the blowout preventer.

Among the other findings, the report said:

There were "no indications" that Transocean had tested intervention systems at the surface, "as was required by Transocean policy", before they were deployed on the well

"Improved engineering rigour, cement testing and communication of risk" by Halliburton could have identified flaws in cement design and testing, quality assurance and risk assessment

A Transocean rig crew and a team described as "mudloggers" working for Halliburton Sperry Sun may have been distracted by what are described as "end-of-well activities" and, as a result, important monitoring was not carried out for more than seven hours

Crew may have had more time to respond before the explosion if they had diverted escaping fluids overboard.

"To put it simply, there was a bad cement job and a failure of the shoe track barrier at the bottom of the well, which let hydrocarbons from the reservoir into the production casing," said outgoing chief executive Tony Hayward.

"The negative pressure test was accepted when it should not have been, there were failures in well control procedures and in the blowout preventer; and the rig's fire and gas system did not prevent ignition," he said.

The blowout preventer that failed was recovered from the Gulf of Mexico on Saturday. It will be placed in the custody of the US Justice Department and examined.

'Lessons for future'

BP's incoming chief executive Bob Dudley said the report proved that the explosion was "a shared responsibility among many entities".

The company said it had accepted all the recommendations in the report, and would implement them worldwide.

"We are determined to learn the lessons for the future and we will be undertaking a broad-scale review to further improve the safety of our operations," Mr Dudley said in a statement.

BP says dealing with the aftermath of the spill has cost $8bn (£5.2bn), and it has already paid out about $399m in claims to people affected by the spill.

A national commission is expected to submit a report to President Barack Obama by January. A Congressional joint investigation will submit a report later than month.

The US justice department is also investigating the disaster.

The Deepwater Horizon explosion according to BP

19 April - 'Bad' cement is pumped down casing to stop gas and oil (hydrocarbons) leaking into wellbore